From DE 10 2006 054 405 A1 a drive-train of a hybrid vehicle is known, whose drive aggregate comprises an internal combustion engine and an electric machine. The internal combustion engine of the drive aggregate and the electric machine of the drive aggregate are connected, with interposition of a planetary gear system, to a transmission of the drive-train, the planetary gear system comprising the elements of a ring gear, a sun gear and a carrier. A first element of the elements of the planetary gear system is coupled to an input shaft of the transmission. A second element of the elements of the planetary gear system is coupled to the electric machine of the drive aggregate. A third element of the elements of the planetary gear system is coupled to the internal combustion engine, namely via a first clutch which, according to DE 10 2006 054 045 A1, is in the form of a friction clutch. Furthermore, from this prior art it is known to couple two elements of the planetary gear system to one another via a second clutch, this second clutch being a friction clutch according to DE 10 2006 054 405 A1. The second clutch, by means of which two of the three elements of the planetary gear system can be coupled, is also referred to as a bridging clutch and when the second clutch is open the drive-train operates in a first operating mode, whereas when the second clutch is closed the drive-train operates in a second operating mode. The first operating mode is called the EDD operating mode or Electro-Dynamic-Drive mode of the electro-dynamic drive system, while in contrast the second operating mode is also called the ISG operating mode or Integrated-Starter-Generator operating mode.
From DE 102 25 249 A1 a method for operating such a drive-train during the starting process is known, such that according to this prior art, in a starting process with the help of the clutch by means of which the two elements of the planetary gear system can be coupled, the clutch is operated with slip so as to support the torque, namely when the electric machine of the hybrid drive cannot deliver a sufficiently high torque during starting. According to this prior art, frictional work is performed at the bridging clutch, which must necessarily be made as a friction clutch, and this results in wear of the bridging clutch, which is disadvantageous.